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This chapter describes methods for the isolation of eukaryotic and prokaryotic genes by screening Escherichia coil expression libraries with antibody probes, using the bacteriophage expression vector λgt 11. Using these procedures, antibodies directed against pure proteins have been used to isolate a large number of genes. These include the yeast genes for RNA (ribo nucleic acid) polymerases I, II, and III, clatherin, topoisomerase II, and ubiquitin. Antibodies made against impure proteins can also be used to isolate genes. For example, the gene for yeast DNA (deoxyribo nucleuc acid) polymerase I was isolated, using an antiserum directed against an impure preparation of that enzyme. This was made possible by (1) using a genomic DNA library and isolating all sequences of the yeast genome that would react with the antibody and (2) identifying the correct DNA polymerase clones among a …